Solid plate surface heating units have been developed prior to this time. However, generally, they have been constructed of heavy cast materials having relatively high thermal mass. Solid plate surface heating units having a relatively high thermal mass have a disadvantage in that they are relatively slow to heat up and slow to cool down as compared with the widely used sheathed heating elements. In overcoming this disadvantage, solid plate surface heating units have been constructed of light weight cast material having a very desirable high thermal conductivity at a relatively low thermal mass such as aluminum. Aluminum is desirable as a plate surface heating unit since head readily diffuses laterally through the plate surface. However, aluminum has a disadvantage in that it also has a relatively low melting point and cannot be used unprotected at temperatures above approximately 850.degree. F.
Accordingly, when aluminum is employed as a plate surface heating unit, it must be protected from temperatures that exceed its melting point. Typical of prior art temperature limiting attempts is shown in U.S. Pat. No. 3,885,128 wherein a plate surface heating unit having a glass-ceramic surface and a layer of aluminum or copper on the underside is limited to an operating temperature below 1300.degree. F., the critical temperature of the ceramic, by introducing a temperature limiting means to the surface unit such that the power to the heating element is cut off if the temperature rises to a predetermined temperature. In this instance, the temperature limiting application is to protect the glass-ceramic member. In other prior art teachings, plate surface units are provided with flash heat switches wherein full wattage is applied to the heating element until the plate surface reached a predetermined temperature at which time a lower wattage is applied to maintain the plate surface at the predetermined temperature.
The principal object of the present invention is to provide a low thermal mass, solid plate surface heating unit where the plate is made of a high thermal conductive material such as aluminum wherein heat readily diffuses laterally through the plate surface.
Another object of the present invention is to provide a control system for maintaining uniform temperature distribution and a rapid heat up including a temperature limiting feature.
Another object of the present invention is to provide a plate surface unit having an even lateral temperature distribution used with cooking utensils having irregular bottoms supported on the plate surface.
A further object of the present invention is to provide an aluminum heating plate having a control with a temperature limiting means that permits fast heat up rate without overheating the aluminum heating plate when used with cooking utensils having different diameters.
A still further object of the present invention is to provide an aluminum plate surface unit with an even temperature distribution that permits a metal sheathed heating element of high watts density to operate at relatively lower sheath temperature due to efficient thermal coupling between the heating element and the aluminum plate.